Welp...got the results from today's water tests:
Alkalinity: 7.0 (YIKES!)
Nitrate: 75-90 (YIKES! twice!)
Nitrite: 0.20 (YIKES! thrice!)
Could these have spiked somehow, related to stirring up the substrate during my previous water change?
Thanks to all who correctly identified the culprits!!!
Did a 10g water change; will do another 5g Tuesday or Wednesday...then another eval next weekend.